Transparent conductive oxide (TCO) films are used in many display device applications where electric fields must be applied to activate picture elements (pixels) and where optical transparency is essential. For example, liquid crystal display substrates often employ parallel strips of TCO material as electrodes. When a pair of such substrates are combined in a display with their opposing TCO electrode strips oriented to form a matrix, the area of the display through which any pair of TCO strips cross defines a pixel. By applying an electric field between a pair of crossed TCO strips, the liquid crystal disposed therebetween may be reoriented. This reorientation affects how light is transmitted through these activated areas. For example, polarized light traveling through a liquid crystal display will be transmitted through activated pixel areas with a polarization perpendicular to that transmitted outside of the activated pixel areas. Polarizers may then be employed so that the display appears dark in the activated pixel areas while light is transmitted through the display elsewhere.
The speed at which pixels can be activated and deactivated depends critically on the conductivity of the TCO electrodes. A shorter "refresh rate," corresponding to the speed at which pixels can be turned on and off, may be required in many applications including those employing large or high resolution displays. Shorter refresh rates may be realized by increasing the conductivity of the TCO electrodes, especially in displays having a high pixel density. Increasing the conductivity of TCO electrodes also enhances the display appearance by improving uniformity.
One way in which to increase the conductivity of a TCO layer is to anneal it at high temperatures (above about 250.degree. C.). When glass is used as the substrate material, this method is viable. However, in many applications such as large area liquid crystal displays, glass substrates are too heavy, and so polymeric substrates are preferred. Polymer materials suitable as substrate materials in liquid crystal displays often have glass transition temperatures and melting temperatures well below the high temperatures required for annealing to increase the conductivity of TCO layers. As such, high temperature annealing is not an available option when attempting to increase the conductivity of the TCO electrodes when polymeric substrates are employed.
Another means of increasing the conductivity of a TCO layer is to provide an auxiliary metal layer in contact with the TCO layer. Typically, the metal layer takes the form of a narrow strip, or line, of metallic material deposited on a TCO electrode. Addition of a metal strip increases the conductivity of a TCO electrode by decreasing the resistivity according to the following relationship: ##EQU1##
where R.sub.T is the resistivity of the electrode as a whole, R.sub.TCO is the resistivity of the TCO layer, and R.sub.M is the resistivity of the metal strip. When R.sub.M is much less than R.sub.TCO, which is typically the case, R.sub.T approaches R.sub.M, thus resulting in an electrode having a resistivity much lower than that of a bare TCO electrode. The increase in conductivity results as long as the metal layer is continuous along the length of the electrode. This is significant because high density displays having small pixels and thereby small electrodes require small auxiliary metal layers that may be amenable to cracks or fractures that disrupt electrical conductivity.
Because transparency of the final device is often essential, and because metal layers thick enough to enhance the TCO electrode conductivity are generally optically opaque, it is important that the metal strip does not substantially cover the TCO electrode. Moreover, when independently addressable TCO electrode strips are arranged in close proximity on a substrate, alignment of each metal strip with each TCO strip is essential. Without alignment, the metal strips may cross over to adjacent TCO electrodes, thereby causing an electrical short across adjacent electrodes. Alignment is especially critical on large area and high resolution displays, where the electrode strips may be longer or closer together, thus leaving less room for error.
Typically, one of three methods (or a variation thereof) are used to fabricate TCO electrodes having auxiliary metal strips. First, thin metal strips may be directly deposited through a mask onto preexisting TCO electrode strips. This requires precision alignment of the deposition mask with the patterned electrodes. Second, metal strips may be deposited on substrate having a TCO layer that has not yet been patterned into electrodes. Portions of the TCO layer and any unwanted metal are then removed to form TCO electrodes with auxiliary metal strips. This requires precision alignment of an etch mask or a laser scribe with the patterned metal strips. Lastly, metal strips may be deposited directly onto a substrate followed by deposition of TCO strips directly on top of the metal strips. Again, this requires precision alignment of a deposition mask with the patterned metal strips. In each of these methods, the required precision alignment step reduces the efficiency of the process and risks introduction of defects.
A method for providing auxiliary metal strips to TCO electrodes without a precision alignment step is described in U.S. Pat. No. 5,342,477 to Cathey. This method involves providing a substrate having a plurality of transparent electrodes, each electrode comprising a strip of transparent silicon dioxide stacked on a strip of transparent conductive material. The entire surface is then coated with a highly conductive material. The highly conductive material is then vertically etched until the material on top of the electrodes is removed and an area between the electrodes is exposed. What remains is a "runner" of conductive material along each side of the transparent electrode stack.
While the method disclosed by Cathey does not require a high precision alignment step, it has major deficiencies affecting its viability. First, the method relies on relatively thick electrode stacks so that conductive material will accumulate at the edges of the electrodes during deposition and remain there after the etching step. While the stack is substantially transparent, it is well known that increasing the electrode thickness will decrease the brightness of the display. Second, the conductive "runners" contact only the sides of the transparent conductive portions of the electrode. Because the transparent conductive portions must be thin, the total area of surface contact between the conductive runners and the transparent conductive strips is quite small. Thus, delamination of the conductive runners from the transparent conductive strips is likely. When delamination occurs, the conductive runners have no effect.
Another method for providing auxiliary metal strips to the edges of TCO electrodes without a precision alignment step is discussed in Japanese Kokai Patent Application No. 4-360124. In the method there disclosed, TCO electrodes are formed on a substrate by conventional photolithography. The photoresist is left on the TCO material, and a metal is electroplated onto the exposed side edges of the electrodes. The photoresist is then removed to leave a series of TCO electrodes having metal strips along their edges. While this method addresses some of the deficiencies of the Cathey method, the reliance on metal plating techniques risks excessive metal build-up between electrodes that would short-out adjacent electrodes in the display. This risk is especially apparent for high density displays where the distance between electrodes may be quite small.